1. Field of the Invention
The present invention relates to a method and an apparatus for pyrolytically decomposing waste plastic, and in particular to a technique for efficiently producing high quality fuel oil by using pyrolysis of waste plastic.
2. Description of the Prior Art
Conventionally, plastic materials have been widely used for manufacturing domestic electric appliances and other various articles because of their advantageous features of light weight and prominent strength, but the cycle of manufacturing and consuming the plastic materials has been resulted in producing a huge amount of waste plastic articles, and such waste plastic articles have required to be burned or buried for settlement at present. More recently, however, since the legislation of a law for realizing recycling use in 1991 in Japan, the public interest in Japan has been increasingly attracted toward the recycling use and re-exploitation of waste plastic products.
Now, the plastic comes in a great variety, and one piece of plastic article is manufactured, in general, by using various kinds of plastic. However, each kind of plastic requires a different treatment method to be employed for the purpose of facilitating the re-exploitation. To this end, it is necessary for the waste plastic articles to be separated into various components in accordance with kinds of plastic involved, but such a work is troublesome, and it also seems practically difficult to perform such a work completely. As a result, it has been inevitable for the quality and value of the recovered products to be reduced because of this incomplete separation. In more difficult cases, it is impossible to put the reclamation into practice.
In such a circumstance, a technique of dry distillation of the waste plastic has attracted much attention, because this technique has a possibility to reclaim the waste plastic articles into a product having a relatively high value. However, such a dry distillation technique requires a high cost at present, and it is also necessary to further improve the recovering efficiency in this technique. Moreover, in a case of dealing with a thermoplastic material such as polystyrene, polypropylene and the like, which has a low heat conductivity, it is difficult to achieve a quick and uniform heating of a large volume of articles made of the thermoplastic, and the oil obtained from such a thermoplastic source contains a great variety of constituents whose boiling points are widely distributed. Accordingly, in a practical application of the dry distillation technique, it is also necessary to improve the quality of the obtained oil.
As a method for improving the oil quality. Japanese Laid-Open Patent Publication (Kokai) Nos. S63-178195 and H2-29492 propose to use a vapour phase reaction catalyst such as zeolite and the like for the purpose of improving the oil quality. However, in the apparatus for carrying out this method, it is necessary to heat the thermal decomposition section and the oil quality improvement section separately. As a result, this apparatus requires a large space for its installation and a high driving cost.
Furthermore, when a waste plastic mixture containing polyvinyl chloride resin (which will be referred as a PVC resin hereinafter) is thermally decomposed, a corrosive hydrogen chloride gas is generated and this damages a wall of the thermal decomposition furnace. In addition, some products resulting from the decomposition of a plasticizing agent contained in the PVC resin often function to choke the gas flow in a piping system of the decomposition apparatus.
As described above, the conventional method for thermally decomposing waste plastic has been associated with problems concerning poor oil quality and recovery yield for the obtained fuel oil and a need to provide a protection against the damage due to the generation of hydrogen chloride and decomposed products from the plasticizer material to the apparatus for thermal decomposition.